Method for preparing a complex of an aluminum alcoholate with aluminum and boron hydrides



United States Patent METHOD FOR PREPARING A COMPLEX OF AN ALUMINUMALCOHOLATE WITHALUMINUM AND BORON HYDRIDES No Drawing. ApplicationJanuary '13, 1958 Serial No. 708,404

Claims. (Cl. 260-448) This invention relates to the preparation ofcomplexes of an aluminum alcoholate with aluminum and. boron hydrideshaving the formula in which R, R and R are monovalent radicals selectedfrom the group consisting of alkyl, cycloalkyl, alkenyl, aralkyl or analkyl radical substituted with a. halogen and in which R, R and R may bethe same or different radicals.

These complexes are useful as reducing agents for reducing variousorganic and inorganic compounds, such as for reducing aldehydes tocarbinols, nitriles to amines and carboxylic acid esters to primarycarbinols. Some of these complexes are crystalline or glassy solids andothers are liquids with a tendency to supercooling. They are stablecompounds and are soluble in a variety of solvents, such as diethylether, benzene, hexane, chloroform, carbon tetrachloride andtetrahydrofuran, the solubility varying with the specific complex andspecific solvent. The thermal stability of these compounds varies, somebeing distillable while others decompose before reaching thedistillation temperature.

One method for preparing these compounds is described in my copendingapplication Serial No. 683,953, filed September 16, 1957. In accordancewith this method aluminum hydride is reacted with an ester of boric acidin a liquid carrier at a temperature between the temperature at whichaluminum hydride decomposes, the preferred temperature being between 0C. and normal room temperature. The reaction is illustrated by thereaction of aluminum hydride with methyl borate as shown by theequation:

One or more moles of methyl borate in the above equation may be replacedby the boric acid ester of a different alcohol.

The present invention provides another method for preparing the abovementioned complexes. In accordance with the method of the presentinvention an aluminum alcoholate is reacted with diborane in a liquidcarrier containing a solvent for diborane at a temperature between about0 C. and about normal room temperature. The aluminum alcoholate may beselected from those having the formulas Al(OR)s, Al(OR Al(OR /OR /ORAlqOR Al OR OR; 0R

and mixtures thereof where R, R and R are monovalent radicals selectedfrom the group consisting of alkyl, cyclo alkyl, alkylene, aralkyl or analkyl radical substituted with a halogen and in which R, R and R may bethe same or diiferent radicals. The reaction is illustrated by the icereaction of diborane with aluminum methylate as shown by the equation:

In the preferred practice of the invention, the aluminum methylate inthe aboveequation may be replaced by another aluminum alkylate, amixture of aluminum alkylates, a mixed molecule aluminum alkylate ormixtures thereof. Suitable liquid carriers are the ethers, such asdiethyl ether, tetrahydrofuran, dibutyl ether or the dimethyl or diethylethers of diethylene glycols, and hydrocarbon solvents, such as hexane,cyclohexane or benzene, or -mixtures of an ether and a hydrocarbonsolvent. If the liquid carrier is not a solvent for diborane, a solventis mixed with the liquid carrier in an amount sufficient to dissolvethediborane.

As illustrative of aluminum alcoholates which may be used in thepractice of the invention, I may mention aluminum ethylate,isopropylate, hexylate, methylate, npropyplate, dimethylmonoisopropylate, n-butylate, tertiary butylate, secondary butylate,n-amylate, tertiary amylate, methyl isobutyl carbinylate, 3-heptylate,di-isopropyl carbinylate, n-octylate, 2,6,8-trimethyl-4-nonylate,allylate, oleylate, stearylate, 2-methyl-2,4-pentanediol alcoholate,benzylate, p-methyl benzylate, o-methyl benzylate, Z-phenylcyclohexylate, l-3-dichloro-2-propylate, cyclohexylate, cyclopentylateand cycloheptylate.

The invention is illustrated further by the following speciflc examples.

Example 1 A solution of diborane in tetrahydrofuran was prepared asfollows. 136 ml. of a lithium aluminum hydride solution in diethyl ethercontaining 7.5 percent lithium aluminum hydride was diluted with 50 ml.of diethyl ether and added to a solution of 47.6 grams of borontrifiuoride etherate in 150 ml. of diethyl ether. The diborane generatedwas absorbed in ml. of tetrahydrofuran. 0.5 ml. of this solution evolved70 ml. of hydrogen on hydrolysis indicating that the whole solutioncontained 2.32 grams of diborane.

34.7 grams of aluminum isopropylate was dissolved in 150 ml. oftetrahydrofuran and this solution was added to the above diboranesolution. A clear solution resulted. There was no apparent reaction orheat evolved. The reaction mixture was allowed to stand for about 10days. The tetrahydrofuran was then distilled out at atmospheric pressureunder a nitrogen atmosphere; then distillation under about 1 mm. vacuumgave a forerun of 10 grams which was a mixture of isopropyl borate andtetrahydrofuran. This was fractionated separately at atmosphericpressure. Then, the product was distilled under a vacuum of about 1 mm.and at a temperature between C. and C. a distillate of 23.3 grams wasobtained. The product analyzed 1.76 percent active hydrogen. Thetheoretical active hydrogen content of the desired product is 1.80percent.

Example 2 Aluminum dimethyl monoisopropylate was prepared in thefollowing manner. 20.4 grams of freshly distilled aluminum isopropylatewas dissolved in 120 m1. of dry benzene. 15 ml. of methanol was added tothe solution and a gel formed. The mixture was refluxed for 30 minutes.The methanol was pulled off by vacuum evaporation leaving a very finepowder.

10.4 grams of this powder was mixed with ml. of tetrahydrofuran. Thesuspension was added to a solution of 0.98 gram of diborane in 39 ml. oftetrahydrofuran. The mixture was stirred for 3 hours at 2025 C. Severaldays later, the solution was evaporated in vacuum and dried at 80 C. atabout 1 mm. pressure to constant weight. The resulting white powderweighed 10.9 grams, the yield being about 96 percent of the theoretical.The powder contained 1.9 percent active hydrogen.

I claim:

1. The method for preparing complexes of aluminum alcoholates withaluminum and boron hydrides having the formula 0 R AIH3(B H103 A1 O R1where R, R and R are monovalent radicals selected from the groupconsisting of alkyl, cycloalkyl, alkenyl, aralkyl and an alkyl radicalsubstituted with a halogen which comprises reacting an aluminumalcoholate with diborane in an inert liquid carrier in which diborane issoluble in the proportion of two moles of diborane to four moles ofaluminum alcoholate, the aluminum alcoholate being selected from thegroup consisting of those having the formulas A1(OR) Al(OR Al(OR /OR /ORAl-OR AI OR OR; OR;

References Cited in the file of this patent UNITED STATES PATENTSSchlesinger Feb. 15, 1949 Schlesinger Feb; 15, 1949

1. THE METHOD FOR PREPARING COMPLEXES OF ALUMINUM ALCOHOLATES WITHALUMINUM AND BORON HYDRIDES HAVING THE FORMULA